1. Field of the Invention
The present invention relates to a direct fuel injection engine provided with a fuel injector which injects a fuel directly into an engine combustion chamber.
2. Related Art
Conventionally, there has been known a direct fuel injection engine provided with a fuel injector which injects a fuel directly into a combustion chamber. In such an engine, a fuel injection is made in a later stage of a compression stroke in a low engine load so that an air fuel mixture is concentrated around an ignition plug so as to accomplish so called a stratification combustion. Consequently, a combustion stability can be obtained while an air fuel ratio or air fuel mixture is kept fuel lean to thereby improve a fuel consumption efficiency.
Further, for instance, as shown in Japanese Patent un-examined publication No. 8-193536, it has been proposed that a separate injection in which a fuel is injected is made in both intake and compression strokes is performed in an engine starting condition, or cranking condition to disperse the air fuel mixture for transmitting the fire in the combustion chamber through the intake stroke injection and to form a combustible air fuel mixture around an ignition plug through the compression stroke injection.
Meanwhile, it is necessary for an automotive engine to improve an emission performance in which a production and an emission of harmful substances, such as HC, CO, and NOx contained in an exhaust gas is suppressed as low as possible.
For this purpose, it has been conventionally done that a catalyst is disposed in an exhaust passage to clean the exhaust gas. As the catalyst, three component catalyst has been generally known to clean HC, CO and NOx in the vicinity of a theoretical air fuel ratio. In addition, a catalyst has been developed to clean NOx in a lean burn operating condition in which a stratification combustion is established.
However, it should be noted that the exhaust gas cleaning catalyst is not able to clean the exhaust gas sufficiently in a cold condition where a catalyst temperature is lower than an activating temperature thereof. Consequently, a lot of HC (and CO) or NOx would be emitted under such condition. Therefore, it is required to reduce the emission of HC and NOx in such cold condition as well as to facilitate a warm up of the engine.
Meanwhile, where the compression stroke injection is made under the cold condition so that the air fuel mixture is controlled to a lean condition to establish the stratification combustion condition in the direct injection engine, it is disadvantageous in that a heat efficiency is facilitated while a warming up of the catalyst is delayed due to the fact that the heat amount supplied to the exhaust system is reduced.
In addition, in the injection system disclosed in the above publication, the separate injection is carried out in the starting condition. This is done for improving a starting performance and not for the emission performance during the cold condition nor for the warming up of the catalyst. Further, due to the intake stroke and compression stroke injections during the starting condition, the injected fuel would adhere to the ignition plug to deteriorate the ignition performance.
In view of this, it would be proposed that the catalyst is disposed close to the engine body, for example, the catalyst is directly connected to the engine body. However, this might deteriorate the catalytic performance due to an undue increase of the temperature of the catalyst during a high speed and high load engine condition in which the exhaust gas temperature is increased. In order to deal with this, the fuel is overly supplied than an air fuel ratio for a desirable output to reduce the exhaust gas temperature by a latent heat of the fuel. As a result, the fuel consumption efficiency during the high speed condition would be deteriorated.
In order to improve the fuel consumption efficiency, it is necessary to dispose the catalyst sufficiently away from the engine body not to unduly increase the catalyst temperature without establishing the over rich air fuel mixture in the high speed and high load condition. However, this would increase a time period before the temperature of the catalyst is sufficiently warmed up after the engine start. Therefore, it is important to improve the emission performance and facilitate the warming up of the catalyst the time period.